The Swiss great advanced as the group winner, with South Africa's Anderson - who was already assured of his place in the last four - joining him as runner-up.

Thiem's finals wait continues

Thiem, 25, has been a regular fixture in the world's top 10 over the past couple of years and, after reaching his first Grand Slam final at the French Open in June, came to the season-ending tournament looking to prove his credentials on a bigger stage once again.

But, before meeting Nishikori, he knew he faced an uphill battle to reach the semi-finals at the O2 Arena for the first time.

After performing well below par in his defeats against Anderson and Federer, Thiem upped his first-serve percentage to put pressure on Nishikori who, like in his loss to Anderson, made a significant number of unforced errors.

The second set was tighter as Nishikori rallied but another break for Thiem put him in command with a 4-3 lead.

Superb court coverage from the Austrian kept him in the point at 30-0 in what proved to be the final game, a brilliant forehand winner bringing up three match points.

And he needed only one, thumping down an ace out wide to seal victory.

"Whatever happens in the night match it is good to get a win here," Thiem said.

"I wasn't thinking about winning in two sets, I wasn't playing good in my other matches, so I just wanted to show my real self and it was way better."